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Magnocellular deficit theories propose that certain cognitive disorders, such as developmental dyslexia, arise from a generalised deficit in the visual dorsal–magnocellular system and its auditory homologue. Psychophysical tasks requiring rapid temporal processing are often used to probe magnocellular function in this context. However, it is not clear whether performance on these various tasks is actually supported by a common substrate. This study investigated whether different putative measures of magnocellular function produce mutually consistent results, and thus evaluated the extent to which they target the same neural mechanisms. Within the PERGENIC project, 1060 participants completed four psychophysical tasks: detection of ‘frequency-doubled’ gratings (FD); detection of pulsed gratings of low spatial frequency on a steady luminance pedestal (SP); detection of coherent motion (CM); and auditory discrimination of temporal order (TO). Although all measures exhibited good test–retest reliability, only the correlation between the two grating detection tasks was of notable magnitude (FD–SP; ρ = .39); other correlations between measures were poor to modest, ranging from ρ = .06 to .20. For each measure, we also identified a group of participants with very low sensitivity, but found only limited consistency between these putative ‘magnocellular deficit’ groups. Our results suggest that performance on each of these measures is primarily determined by a different neural substrate. Accordingly, we recommend against interpreting such psychophysical tasks as general measures of ‘magnocellular function’. [Supported by the Gatsby Charitable Foundation.]